Method of making a finned tube heat exchanger having a circular cross section

ABSTRACT

A method of making heat exchangers in the form of an elongated tubular member, of substantially round transverse cross-section and having outwardly projecting fins thereon, which include initially forming the tubular member into a substantially rectangular transverse cross-sectional shape, forming the fins on the outer faces of the tubular member, and then expanding the tubular member into the aforementioned substantially round crosssectional shape.

United States Pasternak atent 1191 1 METHOD OF MAKING A FINNED TUBE HEATEXCHANGER HAVING A CIRCULAR CROSS SECTION [75] Inventor: Stephen F.Pasternak, Park Ridge,

[73] Assignee: Peerless of America, Incorporated,

Chicago, Ill.

[22] Filed: July 2, 1973 211 App]. No.: 375,764

52 0.5. Ci. 29/1573 A, 29/1573 v, 72/325 51 1111.01 B21d 53/02, 1323p15/26 58 rt q gs arch... 29/1573 A, 157.3 B, 157.3 v;

[56] References Cited UNITED STATES PATENTS Solnick et a1 29/157.3 VKritzenl 29/157.3 A X 1 Feb. 18, 1975 3,222,764 12/1965 Hansson et a1.29/157.3 A 3,692,105 9/1972 OConnor 29/1573 B X 3,781,959 1/1974O'Connor 29/1573 A 3,791,003 2/1974 B Pasternak 29/1573 PrimaryExaminerC. W. Lanham Assistant ExaminerD. C. Reiley, lll Attorney,Agent, or Firm-Root & OKeeffe [5 7] ABSTRACT A method of making heatexchangers in the form of an elongated tubular member, of substantiallyround transverse cross-section and having outwardly projecting finsthereon, which include initially forming the tubular member into asubstantially rectangular transverse cross-sectional shape, forming thefins on the outer faces of the tubular member, and then expanding thetubular member into the aforementioned sub stantially roundcross-sectional shape.

11 Claims, 8 Drawing Figures METHOD OF MAKING A FINNED TUBE HEATEXCI-IANGER HAVING A CIRCULAR CROSS SECTION BACKGROUND OF THE INVENTIONThis invention relates to methods of making heat exchangers, and, moreparticularly, to methods of making heat exchangers of the externallyfinned or spined type, which are substantially round in transversecrosssection.

It is a primary object of the present invention to afford a novel methodof making heat exchangers.

Another object of the present invention is to afford a novel method ofmaking a tubular heat exchanger, which is substantially round intransverse cross-section and has outwardly projecting spines or fins onthe outer periphery thereof.

A further object of the present invention is to afford a novel method ofmaking a substantially round heat exchanger embodying integrally formedspine-type fins.

Heat exchangers, which are susbstantially round in transverse crosssection and embodying integral spinefins on the outer periphery thereofhave been heretofore known in the art, being shown for example in R. W.Kritzer US. Pat. No. 2,247,243 wherein needle-like spines are cut orgouged from the round, tubular wall of the heat exchanger in relativelythin, sharp slivers; in my US. Pat. No. 3,727,682, which discloses around, tubular heat exchanger wherein spines are cut or sliced from theouter periphery of the heat exchanger during relative rotary movementbetween a cutting tool and the tubular member; and R. W. Kritzer US.Pat. No. 3,360,040 wherein longitudinally extending fins on the outerperiphery of such a round tubular member are transversely slit to formthe outwardly projecting spines.

Although spine-type, round, tubular heat exchangers of theaforementioned types heretofore known in the art have been highlysuccessful and have had much commercial success and the methods ofmaking the same have been successful, such methods have had certaininherent disadvantages, such as, for example, the speed of productionrealized therefrom being relatively slow; the number of spines which maybe formed around the periphery of the tubular member being limitedbecause of the necessity of affording space for the spine-formingmechanisms; requiring the expenditure of large sums of money fortooling; requiring different tools for different diameters of tubing; orrequiring machines of desirably large size, and the like. It is animportant object of the present invention to overcome suchdisadvantages.

Methods of making heat exchangers, which comprise successively cuttingor gouging spines or fins from ribs formed on the outer periphery of atubular member, which is substantially rectangular in transverse crosssection, have been heretofore known in the art, being disclosed, forexample, in R. W. Kritzer US. Pat. No. 3,202,212. It is anotherimportant object of the present invention to enable heretofore knownmethods of the aforementioned type to be utilized in a novel andexpeditious manner as a part of a novel method of making outwardlyspined, tubular heat exchangers which are substantially round intransverse cross section.

Another object of the present invention is to afford a novel method ofmaking an externally spined heat exchanger, which is substantially roundin transverse cross section, wherein the spines may be integrally formedon substantially flat surfaces of substantial width.

Yet another object of the present invention is to enable an internallyribbed, outwardly spined heat exchanger, which is substantially round intransverse cross section, to be made in a novel and expeditious manner.

A further object of the present invention is to afford a novel method ofmaking an externally spined tubular heat exchanger, having asubstantially round transverse cross section, which is practical andefficient, and which may be readily and economically used commercially.

Other and further objects of the present invention will be apparent fromthe following description and claims and are illustrated in theaccompanying drawings which, by way of illustration, show the preferredembodiments of the present invention and the principles thereof and whatI now consider to be the best mode in which I have contemplated applyingthese principles. Other embodiments of the invention embodying the sameor equivalent principles may be used and structural changes may be madeas desired by those skilled in the art without departing from thepresent invention and the purview of the appended claims.

DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a perspective view of a heat exchanger, exemplifying thepreferred type of construction afforded by the present invention;

FIG. 2 is an end view of the. heat exchanger-shown in FIG. 1;

FIG. 3 is a fragmentary, perspective view of a tubular member,illustrating an intermediate step in the formation of the heat exchangershown in FIG. 1;

FIG. 4 is a fragmentary longitudinal sectional view taken substantiallyalong the lines 44 in FIG. 3;

FIG. 5 is a somewhat diagrammatic view, illustrating a modified form ofthe present invention;

FIG. 6 is an end view of a tubular member, illustrating a step inanother modified form of the present invention;

.FIG. 7 is an end view, similar to FIG. 2, but illustrating the type ofheat exchanger afforded by the method embodying the step illustrated inFIG. 6; and

FIG. 8 is an end view of a tubular member, illustrating an intermediatestep in another modified form of the present invention.

DESCRIPTION OF THE EMBODIMENTS DISCLOSED HEREIN A tubular heat exchangeror heat transfer element 1 is shown in FIGS. 1 and 2 of the drawings toillustrate a a preferred form of heat transfer element to be afforded bythe preferred method of the present invention.

The heat exchanger 1 is substantially round in transverse cross sectionand embodies an elongated tubular sidewall portion 2, from the outerperiphery of which a plurality of rows 3 of fins in the form ofelongated spines 4 project outwardly. The rows 3, preferably, aredisposed in parallel relation to each other, and extend longitudinallyof the wall portion 3 in parallel relation to the longitudinal axis ofthe latter.

In making the heat exchanger 1 in accordance with the preferred methodof the present invention, a tubular member 5, FIGS. 3 and 4, is firstformed of any suitable material, such asffor example, aluminum, thetubular member having the construction of the end portion 6 of thetubular member 5, shown in FIG. 3. With this construction, it will beseen that the tubular member 5, when it is so formed, is substantiallyrectangular in transverse cross section, and embodies two substantiallyparallel, oppositely disposed sidewall portions 7 and 8, interconnectedat their respective, opposite longitudinal edges by two other oppositelydisposed sidewall portions 9 and 10, which, preferably, are slightlyconvex outwardly, as shown in FIG. 3. The tubular member is of greaterwidth between the sidewall portions 9 and 10 then it is between thesidewall portions 7 and 8. Each of the sidewall portions 7 and 8 has aplurality of elongated ribs 11 projecting transversely, outwardly fromthe outer face thereof, and extending longitudinally thereof inparallel, transversely spaced relation to each other, FIG. 3.

After the tubular member 5 has been so formed, the fins 4 may be formedthereon in the rows 3 by successively, from one end portion of thetubular member A toward the other end portion B thereof, cutting orgouging the rows 3 of fins 4 from the respective ribs 11 in the mannerdisclosed in the aforementioned Kritzer US. Pat. No. 3,202,212. In thisoperation, the spines or fins 4 are formed on each rib 11 by means of asuitable cutting tool, which first cuts along lengthwise of therespective rib to form the surface 12, FIG. 3, and then continues toform the surface I3. The spine thus cut or gouged from the rib is thenbent outwardly preferably to approximately 90 with respect to the planeof the rib. This gives each spine a configuration illustrated in FIGS. 2and 3 wherein it may be said that each spine 4 has a base portion 14,integral with the respective wall portion 7 or 8 to which it isattached, an intermediate thin portion 15 of rectangular cross section,a portion 16 sloping outwardly from the intermediate portion 15 toafford an enlarged portion 17, and another portion 18 sloping inwardlyfrom the enlarged portion 17 and terminating at the outer end of thespine 4 in a thin edge 19, FIG. 4. Although, because of the spacing ofthe fins 4 along the ribs, the spines 4 are initially formed with theenlarged portions 17, the cutting operation causes the fins 4 tocompress longitudinally so that, as a practical matter, the enlargements17 substantiallydisappear on fins of usual thickness, to thereby afforda relatively smooth-sided appearance for the outer ends of the fins 4similar to that shown in FIG. I.

Thereafter, in the practice of the preferred method of the presentinvention, the tubular member 5 may be cut off to the desired length,such as, for example, the length of the heat exchanger 1, FIG. 1, andexpanded into a form wherein the transverse cross-sectional shapethereof is round, or substantially round, to thereby afford the heatexchanger 1 shown in FIGS. 1 and 2. It will be seen that in the expandedform of the original tubular member 5, which affords the heat exchangerl, the sidewall portions 7-10 are disposed in such relation to eachother as to afford the substantially circular shaped sidewall portion 2,with the rows 3 of fins 4 on the sidewall portions 7 and 8 separated bythe arcuate shaped sidewall portions 9 and 10, FIG. 2. The spacing ofthe rows 3 on the sidewall portion 7 from the rows 3 on the sidewallportion 8 may be controlled by controlling the width of the sidewallportions 9 and 10, the spacing being greater with greater widths of thesidewall portions 9 and 10 and being lesser with lesser widths of thesidewall portions 9 and 10.

During the cutting or gouging of the spines 4 from the ribs 1], thetubular member 5 may be internally supported, if desired, such as, forexample, by inserting a supporting block or rod, such as the rod 20shown in broken lines in FIG. 4 thereinto, the rod 20, preferably,complete filling the tubular member 5 and being disposed therein with asnug but freely slidable fit. After the fins 4 have been formed on thetubular member 5, the rod 20 may be removed prior to expansion of thetubular member 5 into the finished form represented by the heatexchanger 1.

The expansion of the tubular member 5 from the shape shown in FIGS. 3and 4 into the shape which affords the substantially round tubularmember 2 of the heat exchanger 1, shown in FIGS. 1 and 2, may beaccomplished in any suitable manner such as, for example, mechanicallyexpanding it by passing a mandrel, such as, for example, an internalexpanding mandrel or draw rod therethrough. However, where the variousfactors, such as, for example, the wall thickness and the material fromwhich the tubular member 5 is constructed, permit, I prefer toeffect-the expansion of the tubular member 5 into the rounded form ofthe tubular member 2 by passing working fluid, such as, for example,compressed air or hydraulic fluid or the like, thereinto at a confinedpressure suitable for effecting such expansion but less than thepressure required to burst the tubular member. For example, with thetubular member 5 made from aluminum, and with the tubular member 2having an inside diameter of l inches and a wall thickness of .040inches, and with 14 ribs or rows of spines 4 disposed around the outerperiphery of the finished tubular member 2, a pneumatic or hydraulicpressure in the nature of from 550 to 750 pounds per square inch issuitable for effectingthe desired expansion of a tubular member of thetype of the member 5, shown in FIG. 3, to a substantially round tubularmember, such as the tubular member 2, shown in FIGS. 1 and 2. Theinitial, slightly convex-outwardly shape of the narrower sidewalls 9 and10 assists in the forming of the round shape of the sidewall 2 in thefinished heat exchanger.

If desired, the fins 4 may be formed on the tubular member 5 at anintermediate portion thereof, which is spaced from the opposite ends ofthe tubular member 5. Subsequently, during the expansion of the tubularmember 5 into the aforementioned substantially round, transverse crosssectional shape, the opposite end portions of the tubular member 5 maybe firmly clamped by suitable means such as clamping dies, or the like,not shown, so as to prevent expansion of the end portions, and workingfluid may then be passed into the tubular member 5 to thereby causeexpansion of the intermediate portion thereof, disposed between theaforementioned opposite end portions. This method is effective toproduce a heat exchanger of the type somewhat diagrammaticallyillustrated in FIG. 5, wherein an elongated heat exchanger la, embodyingtwo end portions 21 and 22, disposed on opposite sides of anintermediate portion 23 is shown. The fins 4 were formed on only theintermediate portion 23 of the tubular member so that the end portions21 and 22 of the finished heat exchanger la have smooth outerperipheries. In this latter type of construction, the end portions 21and 22 afford highly practical members for quickly and easily connectingthe heat exchanger la to headers, to other tubular members, or to anaccumulator, and the like.

In FIG. 6, a tubular member 5b is shown which comprises a modified formof the original tubular member 5, as represented by the end portion 6 ofthe tubular member 5 shown in FIG. 3. The tubular member 5b is identicalin construction to the aforementioned tubular member 5 except that thetubular member 5b embodies a plurality of ribs 24 extending between thesidewall portions 7 and 8 and disposed in substantially parallel, spacedrelation to each other between the sidewalls 9 and 10. The internal ribs24 extend longitudinally of the tubular member 5b throughout the fulllength thereof, and preferably embody weakened portions 25 midwaybetween the sidewall portions 7 and 8. The tubular member 5b may beformed in any suitable manner such as, by extruding the same. and theweakened portions 25 may be formed therein by reducing the thickness ofthe aforementioned mid portion of the ribs 24 to a thicknesssubstantially less than the thickness of the remainder thereof.

With the tubular member, which is formed as the first step of the methodof making a heat exchanger in the practice of the present invention,constructed in the manner of the tubular member 5b, shown in FIG. 6,effective internal support is afforded for the sidewall portions 7 and 8during the forming of fins 4 on the ribs 1 1. Preferably, the weakenedportion 25 is of sufficiently narrow thickness that the internal ribs 24may be servered therealong merely by the application of the pressureafforded by the working fluid fed into the tubular member 5b, after thespines have been formed on the ribs 11 thereof, in the same manner ashereinbefore described with respect to tubular member 5, shown in FIG.3. However, if desired, the internal ribs 24 may be slit along themid-portions thereof after the fins 4 have been formed on the tubularmember 5b, and before the latter is expanded by the passing of workingfluid thereinto.

As is illustrated in FIG. 7, the heat exchanger which results from themethod including the formation of a tubular member such as the tubularmember 5b, FIG. 6, is similar in construction to the heat exchangershown in FIG. 2, but instead of having an unobstructed passagewaytherethrough, embodies internal ribs 26, which are afforded by thesevered portions of the ribs 24.

In FIG. 8, an intermediate step in another modified form of method ofthe present invention is shown. In the modified method illustrated inFIG. 8, after the original tubular member 5, with the ribs 11 disposedthereon has been formed, and prior to the formation of the spines 4, thetubular member 5 is flattened from the form shown in FIG. 3 to a formsuch as that shown in FIG. 8, wherein the sidewall portions 7 and 8 aredisposed in close adjacent relation to each other and the sidewalls 9and 10 are bowed outwardly. Such flattening of the tubular member 5 maybe accomplished in any suitable manner such as, for example, passing thesame between rollers having ribs thereon, which mate with the spacesbetween the ribs 11. Such flattening of the tubular member 5 preferablyis accomplished by the cold working thereof so as not to effect anywelding together of the sidewall portions 7 and 8.

Preferably, in this method, the flattening is carried out to such anextent that the sidewall portions 7 and 8 are disposed in abuttingengagement with each other, or, at least, to an extent that thepassageway through the tubular member 5 is reduced to such a narrow slitthat support is afforded between the adjacent sidewall portions 7 and 8during the subsequent operation, wherein the fins 4 are cut of gougedfrom the ribs ll.

As will be appreciated by those skilled in the art, although it ispreferred that the flattening of the tubular member 5, during the methodillustrated in FIG. 8, is of such magnitude that the aforementionedsupport is afforded between the sidewall members 7 and 8, if theflattening is not carried out to such an extent, suitable support forthe sidewall members 7 and 8 during the fin-cutting operation, may beafforded by passing a block or rod, not shown, of suitable dimensionsinto the tubular member 5, in a manner similar to that in which the rod29 is shown inserted into the tubular member 5 in FIG. 4, to afford thedesired support for the sidewall portions 7 and 8 during forming of thefins 4.

It will be remembered that during the flattening operation of thetubular member 5, the sidewall portions 9 and 10 are bowed outwardly, asshown in FIG. 8. This is true whether the flattening operation iscarried out to the greatest extent possible or to a lesser extent. Ineither event, the sidewall portions 9 and 10 have been changed from asubstantially flat form, as shown in FIG. 3, to a convex-outwardlyshape, as shown in FIG. 8, so that during subsequent expansion of thetubular member 5, from the shape shown in FIG. 8 to the substantiallyround configuration, such as illustrated in FIGS. 1 and 2, the sidewallportions 9 and 10 are more readily converted into a roundedconfiguration, which is desirable in order to afford, or at leastapproach a true circular configuration for the cross sectional shape ofthe tubular sidewall portion 2 of the finished heat exchanger.

Preferably, in the practice of the last mentioned method of the presentinvention, wherein the tubular member 5 is flattened, as illustrated inFIG. 8, after such flattening of the tubular member 5, the rows 3 of thefins 4 are formed on the ribs 11 in the same manner as hereinbeforedescribed with respect to the method wherein expansion is effected of atubular member from the shape shown in FIG. 3.

Preferably, in the expansion of the tubular member 5 from flattenedcondition, such as shown in FIG. 8, a suitable tool such as, forexample, a wedge-shaped tool, not shown, is inserted into one endportion of the tubular member 5 for a suitable distance such as, forexample 2 to 3 inches, to thereby open the end portion sufficiently tofacilitate the subsequent expansion of the tubular member. Thispreliminary expansion of the one end portion of the flattened tubularmember 5 may be of any suitable amount, such as, for example, thatsufficient to move the end portions of the sidewalls 7 and 8 away fromeach other a distance of between V; and inch.

The subsequent expansion of the flattened tubular member 5, after theaforementioned initial opening or expansion of the one end portionthereof may be effected mechanically or by the use of pressurizedworking fluid as previously discussed with respect to the methodillustrated in FIGS. 1-4. If it is to be performed mechanically,preferably it is accomplished by the passage of a suitable expandingmandrel, such as, for example, a bullet-shaped mandrel, which may beinserted into the preliminarily expanded one end portion of the tubularmember, and then passed onwardly through the tubular member 5. Ifworking fluid such as, for example, compressed air or hydraulic fluid isused for expanding the tubular member 5, it may be fed thereinto throughthe aforementioned one end thereof.

Upon completion of the expansion of the tubular member 5, by the methodwhich includes flattening thereof, as illustrated in PK]. 8, a heatexchanger similar to the heat exchanger as shown in FIGS. 1 and 2, andwhich is round or substantially round in cross section, is afforded.

From the foregoing, it will be seen that the present invention affords anovel method of making an externally spined tubular heat exchanger whichis substantially round in transverse cross section.

Also, it will be seen the present invention affords a novel method ofthe aforementioned type which is commercially practical and efficient.

Thus, while I have illustrated and described the preferred embodimentsof my invention, it is to be understood that these are capable ofvariation and modification, and I therefore do not wish to be limited tothe precisedetails set forth, but desire to avail myself of such changesand alterations as fall within the purview of the following claims.

I claim:

I. The method of making a heat transfer element comprising a. forming anelongated tubular member having 1. a transverse width in one directionwhich is greater than the width thereof in the direction perpendicularto said one direction,

2. oppositely disposed outer wall portions extending along said firstmentioned width, and

3. transversely spaced, outwardly projecting elongated ribs on the outersurfaces of said wall portions and extending longitudinally of saidtubular member b. successively, longitudinally of said tubular member,cuttingfins from said ribs,

0. turning said fins outwardly into outwardly projecting relation tosaid wall portions, and

d. then expanding said-tubular member into a more rounded transversecross-sectional shape.

2. The method of making a heat transfer element comprising a. forming anelongated tubular member having 1. a substantially rectangulartransverse crosssectional shape,

2. two oppositely disposed, substantially parallel walls extending thewidth of said tubular member, and

3. a plurality of transversely spaced, elongated ribs projectingoutwardly from said walls and extending longitudinally of said tubularmember,

b. successively, longitudinally of said tubular member, cutting finsfrom said ribs,

c. turning said fins outwardly into outwardly projecting relation tosaid walls, and

d. expanding said tubular member, with saidvfins thereon, into asubstantially round, internal, transverse cross-sectional shape.

3. The method as defined in claim 2, and in which a. said forming ofsaid tubular member includes forming a plurality of internal ribstherein, which I. extend longitudinally thereof, and

2. extend between said two walls in substantially parallel relation toeach other, and

b. said expanding of said tubular member includes separating each ofsaid internal ribs into two spaced parts.

4. The method as defined in claim 3, and in which a. said forming ofsaid internal ribs includes forming the portions thereof which aredisposed substantially midway between said two walls of reducedthickness as compared to the remainder of said ribs.

5. The method as defined in claim 2, and which includes a. flatteningsaid tubular member into position wherein said two walls are disposed inmore closely adjacent relation to each other prior to said cutting ofsaid fins.

6. The method as defined in claim 5, and in which a. said flattening ofsaid tubular member is performed to an extent wherein, during thesubsequent cutting of fins from said ribs, said two walls are disposedin abutting engagement with each other.

7. The method of making a heat transfer element comprising a. forming anelongated tubular member having 1. a substantially rectangulartransverse crosssectional shape.

2. two oppositely disposed, substantially parallel walls extending thewidth of said tubular member, and

3. a plurality of transversely spaced elongated ribs projectingtransversely outwardly from each of said walls and extendinglongitudinally of said tubular member,

b. successively, longitudinally of said tubular member, cuttingelongated spines from said ribs,

c. turning said spines outwardly into outwardly projecting relation tosaid walls, and

d. passing expanding means into said tubular member and therebyexpanding said tubular member into a substantially round transversecross-sectional shape, with said spines projecting outwardly and formingthe outermost peripheral portion thereof.

8. The method defined in claim 7, and in which a. said expanding meanscomprises a mandrel.

9. The method defined in claim 7, and

a. whichincludes flattening said tubular member into position whereinsaid two walls are disposed in more closely adjacent relation to eachother prior to said cutting of said spines, and

b. in which 1. said expanding means includes a tool,

2. said tool is passed into one end portion of said tubular member tocause initial expansion thereof, and v 3. thereafter other expandingmeans is passed into said tubular member to effect final expansionthereof.

10. The method defined in claim 7, and in which a. said expanding meanscomprises working fluid.

11. The method defined in claim 10, and which includes a. clampinglyholding spaced opposite end portions of said tubular member during saidpassing of said working fluid into said tubular member to therebyrestrain said end portions against said expansion and thereby limit saidexpansion to the portion of said tubular member disposed between saidend portions.

1. The method of making a heat transfer element comprising a. forming anelongated tubular member having
 1. a transverse width in one directionwhich is greater than the width thereof in the direction perpendicularto said one direction,
 2. oppositely disposed outer wall portionsextending along said first mentioned width, and
 3. transversely spaced,outwardly projecting elongated ribs on the outer surfaces of said wallportions and extending longitudinally of said tubular member b.successively, longitudinally of said tubular member, cutting fins fromsaid ribs, c. turning said fins outwardly into outwardly projectingrelation to said wall portions, and d. then expanding said tubularmember into a more rounded transverse cross-sectional shape. 2.oppositely disposed outer wall portions extending along said firstmentioned width, and
 2. said tool is passed into one end portion of saidtubular member to cause initial expansion thereof, and
 2. two oppositelydisposed, substantially parallel walls extending the width of saidtubular member, and
 2. extend between said two walls in substantiallyparallel relation to each other, and b. said expanding of said tubularmember includes separating each of said internal ribs into two spacedparts.
 2. two oppositely disposed, substantially parallel wallsextending the width of said tubular member, and
 2. The method of makinga heat transfer element comprising a. forming an elongated tubularmember having
 3. transversely spaced, outwardly projecting elongatedribs on the outer surfaces of said wall portions and extendinglongitudinally of said tubular member b. successively, longitudinally ofsaid tubular member, cutting fins from said ribs, c. turning said finsoutwardly into outwardly projecting relation to said wall portions, andd. then expanding said tubular member into a more rounded transversecross-sectional shape.
 3. The method as defined in claim 2, and in whicha. said forming of said tubular member includes forming a plurality ofinternal ribs therein, which
 3. a plurality of transversely spaced,elongated ribs projecting outwardly from said walls and extendinglongitudinally of said tubular member, b. successively, longitudinallyof said tubular member, cutting fins from said ribs, c. turning saidfins outwardly into outwardly projecting relation to said walls, and d.expanding said tubular member, with said fins thereon, into asubstantially round, internal, transverse cross-sectional shape.
 3. aplurality of transversely spaced elongated ribs projecting transverselyoutwardly from each of said walls and extending longitudinally of saidtubular member, b. successively, longitudinally of said tubular member,cutting elongated spines from said ribs, c. turning said spinesoutwardly into outwardly projecting relation to said walls, and d.passing expanding means into said tubular member and thereby expandingsaid tubular member into a substantially round transversecross-sectional shape, with said spines projecting outwardly and formingthe outermost peripheral portion thereof.
 3. thereafter other expandingmeans is passed into said tubular member to effect final expansionthereof.
 4. The method as defined in claim 3, and in which a. saidforming of said internal ribs includes forming the portions thereofwhich are disposed substantially midway between said two walls ofreduced thickness as compared to the remainder of said ribs.
 5. Themethod as defined in claim 2, and which includes a. flattening saidtubular member into position wherein said two walls are disposed in moreclosely adjacent relation to each other prior to said cutting of saidfins.
 6. The method as defined in claim 5, and in which a. saidflattening of said tubular member is performed to an extent wherein,during the subsequent cutting of fins from said ribs, said two walls aredisposed in abutting engagement with each other.
 7. The method of makinga heat transfer element comprising a. forming an elongated tubularmember having
 8. The method defined in claim 7, and in which a. saidexpanding means comprises a mandrel.
 9. The method defined in claim 7,and a. which includes flattening said tubular member into positionwherein said two walls are disposed in more closely adjacent relation toeach other prior to said cutting of said spines, and b. in which
 10. Themethod defined in claim 7, and in which a. said expanding meanscomprises working fluid.
 11. The method defined in claim 10, and whichincludes a. clampingly holding spaced opposite end portions of saidtubular member during said passing of said working fluid into saidtubular member to thereby restrain said end portions against saidexpansion and thereby limit said expansion to the portion of saidtubular member disposed between said end portions.